The ban on commercial shellfish harvesting in part of the Marlborough Sounds remains indefinitely as tests show biotoxin levels there have now risen to double the allowable limit.
The area was closed for harvesting last Friday following the discovery of diarrhetic shellfish poisoning (DSP) biotoxins above the allowable limit at WhangakokoBay, on the eastern base of the peninsula known as the "Tongue", in Port Underwood.
Subsequent tests on shellfish there show DSP levels have risen from 21 micrograms (one-thousandth of a gram) per 100g of flesh to 40.
The allowable level is 20 micrograms.
Levels have also risen at two other sites in the area, from two micrograms to seven at Horahora-Kakahu Island and from two to 16 at Opihi Bay.
However, Blenheim's Wairau Hospital senior health protection officer, Al Campbell, said the levels were still negligible.
"We are talking very small amounts."
Mr Campbell was not concerned by the increases, which are characteristic of bloom-type activity.
"They [levels] come and go. They build up and decline.
"It could all change again depending on the weather," he said.
The ban will not be lifted until levels are consistently below the allowable limit.
About 35 mussel farms in the area have been affected.
Mr Campbell said the problem was still isolated and small when compared with the 1800 marine farms in the Marlborough Sounds.
Signs have been posted in the area warning recreational shellfish collectors about the dangers.
The area is a popular customary fishing spot. Mr Campbell said he had been in contact with local iwi about the problem, but could not enforce any ban of individuals collecting shellfish.
Included in the ban are scallops, tuatua, cockles, oysters, tuatua mussels, pipis, catseyes, pupu, and kina or sea-eggs.
Crayfish, crabs, paua and fin fish could be eaten provided the gut was removed, Mr Campbell said.
He said cooking did not remove the toxins from affected shellfish and therefore did not make them safe to eat.
The symptoms of DSP are rapid onset of abdominal pain, diarrhoea, nausea and vomiting, usually within 24 hours of eating the shellfish.
